1. Field of the Invention
This invention relates generally to the field of ground proximity warning systems, particularly to systems that warn of an excessive aircraft altitude loss after take-off or doing a go-around after a missed approach, and even more particularly, to systems wherein the warning criteria are altered as a function of elapsed time after take-off as well as the altitude above ground at which the aircraft is flying.
2. Description of the Prior Art
Ground proximity warning systems that warn of aircraft descent after take-off or during a go-around after a missed approach are known. Examples of such systems are disclosed in U.S. Pat. Nos. 3,946,358; 3,947,808; 3,947,810; 4,319,218 and 4,433,323 assigned to the assignee of the present invention. The systems disclosed in the '358 and '808 patents provide a warning if the descent rate of the aircraft exceeds a predetermined rate below a given altitude, and the systems disclosed in the '810; '218 and '323 patents generate a warning if the altitude loss exceeds a predetermined value before a predetermined altitude is reached. The '323 patent also discloses a system that switches between modes based on time and radio altitude.
While these systems provide a way to warn the pilot of an aircraft of a hazardous condition resulting from an excessive sink rate or from an excessive altitude loss after take-off or missed approach, these systems do not take into account the length of time that the aircraft has been flying in determining the warning envelopes. Consequently, such systems are not entirely suitable for aircraft whose operational characteristics are such that they are normally flown at low altitudes after take-off, because such systems can generate nuisance warnings during intentional low level maneuvering. In addition, many aircraft have radio altimeters that provide unreliable readings immediately after takeoff, and systems that rely solely on a radio altimeter for altitude information are generally inhibited until the radio altitude readings stabilize.